Add more information and refactor format of Avatar API topic

doc/api/avatar.md

 # Avatar API
 
-> [Introduced][ce-19121] in GitLab 11.0
+> [Introduced](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/19121) in GitLab 11.0.
 
 ## Get a single avatar URL
 
-Get a single avatar URL for a given email address. If user with matching public
-email address is not found, results from external avatar services are returned.
-This endpoint can be accessed without authentication. In case public visibility
-is restricted, response will be `403 Forbidden` when unauthenticated.
+Get a single [avatar](../user/profile/index.md#profile-settings) URL for a user with the given email address.
 
-```
+If:
+
+- No user with the given public email address is found, results from external avatar services are
+  returned.
+- Public visibility is restricted, response will be `403 Forbidden` when unauthenticated.
+
+NOTE: **Note:**
+This endpoint can be accessed without authentication.
+
+```text
 GET /avatar?email=admin@example.com
 ```
 
+Parameters:
+
 | Attribute | Type    | Required | Description                                                                                                                             |
-| --------- | ------- | -------- | --------------------- |
-| `email`   | string  | yes      | Public email address of the user |
-| `size`    | integer | no       | Single pixel dimension (since images are squares). Only used for avatar lookups at `Gravatar` or at the configured `Libravatar` server |
+|:----------|:--------|:---------|:----------------------------------------------------------------------------------------------------------------------------------------|
+| `email`   | string  | yes      | Public email address of the user.                                                                                                       |
+| `size`    | integer | no       | Single pixel dimension (since images are squares). Only used for avatar lookups at `Gravatar` or at the configured `Libravatar` server. |
+
+Example request:
 
-```bash
-curl https://gitlab.example.com/api/v4/avatar?email=admin@example.com
+```sh
+curl https://gitlab.example.com/api/v4/avatar?email=admin@example.com&size=32
 ```
 
 Example response:
 
 ```json
 {
-  "avatar_url": "https://www.gravatar.com/avatar/e64c7d89f26bd1972efa854d13d7dd61?s=80\u0026d=identicon"
+  "avatar_url": "https://www.gravatar.com/avatar/e64c7d89f26bd1972efa854d13d7dd61?s=64&d=identicon"
 }
 ```
-
-[ce-19121]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/19121